EP3259700B1 - Wireless card reader - Google Patents

Description

The invention relates to a method for operating a card reader of card units that can be read without contact, such a card reader and card units that can be read without contact.

Card reading devices and cards that can be read without contact are known from the prior art, which comprise an integrated circuit in which information is stored or can be stored. In one type of card readers and cards that can be read without contact, the card reader generates a high-frequency magnetic field in a close range. The integrated circuit of the card that can be read contactlessly is connected to a receiving circuit or a receiving antenna, which is generally designed as an inductor. If the card that can be read without contact is placed in the vicinity of the card reader, the high-frequency magnetic field induces a current in the receiving circuit of the card that can be read without contact, which current is used to supply the integrated circuit with electrical energy. By modulating the generated high-frequency magnetic field, the card reading device can transmit or transfer information to the card that can be read without contact, i.e. to the integrated circuit of this card that can be read without contact. The integrated circuit of the card that can be read without contact is able to switch on a load or capacity of the receiving circuit periodically, which changes the high-frequency magnetic field. The card reading device is designed to detect and evaluate such changes in the magnetic field. In this way, information can be exchanged from the contactless readable card to the card reader. This change in the magnetic field is also referred to as modulation.

A possible embodiment of such card readers and cards that can be read contactlessly is described and standardized in the ISO/IEC 14443 standard “Identification cards—contactless integrated circuit(s) cards—proximity cards”. In the standard, such cards that can be read without contact are also referred to as proximity integrated circuit cards or proximity cards (PICC) for short. The identification cards standardized by the International Civil Aviation Organization - ICAO, which include, for example, electronic passports, electronic identification cards, etc., also work on the basis of this standard. There are also credit cards that meet this standard or use very closely related ones for communication. Cards for a variety of other applications are possible.

In the ISO/IEC 14443 standard mentioned, part 2 "radio frequency power and signal interface" describes different types of transmission. With a type of transmission known as Type B, information is transmitted from the card reader to the contactless readable card that is in the vicinity of the card reader by varying the magnetic field strength of the generated high-frequency magnetic field between an unmodulated magnetic field strength, which is associated with the letter a, and a modulated magnetic field strength , which is associated with a letter b, is shift-keyed. The unmodulated magnetic field strength and the modulated magnetic field strength are different from zero. Combinations of unmodulated and modulated magnetic field strength represent the different symbols of communication from which the logical states 1 and 0 are derived. Shift keying takes place on a time basis, i.e. with an information transmission frequency which, according to the standard, for example, corresponds to one 128th of the carrier frequency of the high-frequency magnetic field. The carrier frequency is set at 13.56 MHz +/- 7 kHz according to the standard.

In order to ensure reliable signal transmission between the card reader and the card that can be read contactlessly, it is necessary for the unmodulated magnetic field strength and the modulated magnetic field strength to be in a predetermined ratio to one another. A so-called modulation index m is defined for this purpose in the standard. This results from the quotient of the difference between the unmodulated magnetic field strength and the modulated magnetic field strength and the sum of these two magnetic field strengths. Expressed in a formula, this means: $$m=\frac{\mathrm{a}-\mathrm{b}}{\mathrm{a}+\mathrm{b}}.$$

In order to ensure reliable detection of the modulation signal, ie the transmitted information, by the contactless readable card, the modulation index must be specified. According to the ISO/IEC 14443 standard, the modulation index should be between 8% and 14%, for example. For the identification cards standardized by the ICAO, there is even a restriction to a value range of 10% to 14%. If the default value for the modulation index is not met by the card reader, this often means that communication with a contactless readable card in the vicinity of the card reader cannot be carried out successfully. Furthermore, it has been shown in practice that a modulation index that actually occurs in the high-frequency magnetic field depends on external influences, for example temperature, materials in the vicinity of the contactless readable card, intrinsic parameters of the contactless readable card, for example its receiving circuit and /or whose integrated circuit, ie microchip, etc. is dependent. A magnetic field strength that is as high as possible with a correctly adapted modulation index is therefore necessary for data transmission.

The same or similar also applies to other modulation methods and transmission or protocol standards.

the DE 10 2009 009 846 A1 describes a card reader and a method with which a modulation index measured in the card reader is controlled and approximated to a default value.

the U.S. 2010/0144269 A1 shows a method and a device for controlling wireless data transmission interfaces of two components, wherein a monitoring device of one component monitors a change in the environment and, when such a change occurs, puts the interface of one component into operation in order to wirelessly exchange data with the interface of another component . The interface for data transmission is switched off while the environment is being monitored.

Contactless readable cards and data stored therein are increasingly being used for cryptographic purposes or to identify the respective bearer of the contactless readable card. The duration in which a communication connection between the card reader and the card that can be read without contact is maintained increases significantly and is usually dependent on processes that are not executed in the card reader or in the card that can be read without contact, but instead, for example, in one with the Card reader communicatively connected bank computer during a bank transaction or in a computer of a police authority that is connected to the card reader by communication technology when querying a database of unwanted persons in the course of a border crossing.

In particular, if the card reading device is designed to be mobile or is only operated using a limited energy store, for example an accumulator or a battery, the operating time of the card reading device is severely limited.

The invention is therefore based on the technical problem of creating a card reader and a method for operating a card reader and a card unit that can be read contactlessly, with which the card reader can be used for a longer period of time and still be reliable and secure communication without restricting the functionality of the system consisting of a card reader and a card unit that can be read contactlessly.

The technical problem is solved according to the invention by the products and methods defined in the independent claims. Advantageous embodiments of the invention result from the dependent claims.

basic idea of the invention

The invention is based on the idea of reducing the magnetic field strength of the high-frequency field in those phases in which the communication connection is "only maintained", since other higher-level processes or applications process the data transmitted from the contactless card unit to the card reader. Without any knowledge of the higher-level process or processes, this can be assumed in those situations in which the contactlessly readable card unit has sent response information to a request from the card reader. As a rule, in the case of near-field communication, communication is always carried out according to a master-slave method (master-slave method), with the card reader being the master (master). The slave is the card unit that can be read contactlessly. Communication is always initiated by the master. After receiving a response to a request in the form of a so-called data frame, it is up to the master to decide when a new communication will take place. In particular, when an application that is connected to the card reader and processes data from the contactless card unit, "gaps" often arise in the communication. In these situations, the energy requirement of the card that can be read contactlessly is minimal if the units required for an analysis of the transmitted data, ie demodulation and further processing, cryptographic processing or modulation for sending back response information, are deactivated or not used. However, some information relating to the communication, in particular negotiated transmission parameters, but also other data, such as exchanged cryptographic keys etc., must remain stored unchanged so that the logical and/or physical communication connection is not interrupted and a functionality of the overall system from the card reader and the application, which is executed, for example, on a PC with a window-based operating system, and a contactless readable card unit is not affected. In order to achieve this, it is proposed that in a period of time between receipt of a data frame sent from the contactless readable card unit (PICC) to the card reader (PCD), the at least due to the received modulation, which is due to the variation of the high-frequency magnetic field through the contactless readable card unit (PICC) is effected, has a correct structure, and a subsequent transmission of information from the card reader (PCD) to the contactless readable card unit (PICC), the magnetic field strength is lowered compared to an initial magnetic field strength, the initial field strength being the magnetic field strength with which the Card reader non-modulated high-frequency magnetic field is generated during information transfer between the card reader and the contactless readable card unit. In this way, a very considerable energy saving can be achieved in the case of long external processing processes for the exchanged data.

definitions

Communication reliability is a measure of whether communication between the card reader and a card that can be read contactlessly is established and maintained for as long as required when the latter is in the vicinity of the card reader. Communication security is a measure of whether the information exchanged during communication is transmitted correctly.

A card that can be read without contact is a card-shaped unit with an electronic circuit that can be operated in a high-frequency magnetic field generated by a card reader in such a way that information can be transmitted between the card reader and the electronic circuit of the contactless readable card via modulation of the high-frequency magnetic field in the vicinity of a card reader card is possible. As a rule, electronic circuitry is designed to switch a load or a capacitance of an oscillating circuit in a targeted manner in order to vary the high-frequency magnetic field and thereby transmit information to the card reader.

A card unit that can be read contactlessly is an entity that has the functionality of a card that can be read contactlessly, regardless of whether the entity is in the form of a card or not.

An entity is referred to as card-shaped if it is in the form of a flat body, the dimensions of which in a planar extent are greater than a thickness perpendicular to the planar extent. In particular, bank cards, credit cards, passport cards, driver's license cards, ID cards, etc. constitute card-shaped entities. All bodies that meet the requirements of the formats ID-1 to ID-3 or ID-000 specified in the ICAO Document 9303 Part 1 standard , are considered map-shaped.

A mode in which the contactlessly readable card unit is operated in a high-frequency magnetic field with a field strength below a magnetic field strength provided for a normal mode is referred to as the low-energy mode of a contactlessly readable card unit.

A card unit that can be read contactlessly is referred to as correctly functioning in low-energy mode, which stores at least the transmission parameters that have been negotiated for a current communication connection and/or other data specific to the communication connection in the low-energy mode in such a way that they are then available again in a normal mode without requiring parameter or data renegotiation.

Near-field communication is communication that uses the electromagnetic near-field of a transmitting antenna to transmit information. Irrespective of an antenna shape, it is understood here as communication in a distance range from the transmitting antenna, which includes distances s from the transmitting antenna that are smaller, usually smaller by more than an order of magnitude, than the wavelength of the electromagnetic field λ divided by 2π ( s<λ/(2π)).

A data frame is an information unit which is exchanged coherently via a modulation of the high-frequency magnetic field in short-range communication between the card reading device and the contactless readable card unit. The structure of such a data frame is in a protocol, which is for the Information transmission is used, set. Depending on the content of the transmitted information, the structure can change. In addition to the "pure information", a data frame usually also includes other components such as a start sequence, an end sequence, status information, error correction information, etc., to name just a few.

A structure of a data frame that is correct due to a received modulation is a structure in which all data fields belonging to a data frame are present according to an evaluated received modulation, i.e. after demodulation, regardless of whether their content makes sense. In relation to the ISO/IEC 14443 family of standards, the structure of a data frame is standardized in Part 3.

Preferred Embodiments

• Erzeugen eines Hochfrequenzmagnetfelds in einem Nahbereich des Kartenlesegeräts (PCD), wobei das Hochfrequenzmagnetfelds gesteuert wird, um eine Kommunikation mit einer kontaktlos auslesbaren Karteneinheit (PICC) auszuführen, und wobei bei einer Informationsübermittlung von dem Kartenlesegerät (PCD) zu einer in dem Nahbereich befindlichen kontaktlos auslesbaren Karteneinheit (PICC) eine Modulation des Hochfrequenzmagnetfelds ausgeführt wird;
• Auswerten des Hochfrequenzmagnetfelds, um eine Variation des Hochfrequenzmagnetfelds durch die in dem Nahbereich befindliche kontaktlos auslesbare Karteneinheit (PICC) zu erfassen und zu analysieren,
• wobei in einem Zeitbereich zwischen einem Empfang eines von der kontaktlos auslesbaren Karteneinheit (PICC) zu dem Kartenlesegerät (PCD) gesendeten Datenrahmens, der zumindest aufgrund der empfangenen Modulation, welche über die Variation des Hochfrequenzmagnetfelds durch die kontaktlos auslesbare Karteneinheit (PICC) bewirkt ist, eine korrekte Struktur aufweist, und einer nachfolgenden Informationsübermittlung von dem Kartenlesegerät (PCD) an die kontaktlos auslesbare Karteneinheit (PICC) die Magnetfeldstärke gegenüber einer Ausgangsmagnetfeldstärke auf eine vorfestgelegt abgesenkte Magnetfeldstärke abgesenkt wird, wobei die Ausgangsmagnetfeldstärke jene Magnetfeldstärke ist, mit welcher das seitens des Kartenlesegeräts nicht modulierte Hochfrequenzmagnetfeld während einer Informationsübermittlung zwischen dem Kartenlesegerät und der kontaktlos auslesbaren Karteneinheit erzeugt wird. Beispielsweise wird nach einem Übermitteln einer Information an die kontaktlos auslesbare Karteneinheit (PICC) und einem anschließenden korrekten Empfang einer Antwort von der kontaktlos auslesbaren Karte, welche über die Variation des Hochfrequenzmagnetfelds durch die kontaktlos auslesbare Karteneinheit (PICC) bewirkt ist, die Magnetfeldstärke gegenüber einer Ausgangsmagnetfeldstärke abgesenkt, mit welcher das seitens des Kartenlesegeräts nicht modulierte Hochfrequenzmagnetfeld während einer Informationsübermittlung zwischen dem Kartenlesegerät und der kontaktlos auslesbaren Karteneinheit erzeugt wird.

A method for operating a card reader (PCD) is proposed, which comprises the steps:

• Generating a high-frequency magnetic field in a close range of the card reader (PCD), the high-frequency magnetic field being controlled in order to communicate with a contactless readable card unit (PICC), and wherein when information is transmitted from the card reader (PCD) to a contactless readable located in the close range card unit (PICC) a modulation of the high-frequency magnetic field is carried out;
• Evaluation of the high-frequency magnetic field in order to detect and analyze a variation of the high-frequency magnetic field by the contactless readable card unit (PICC) located in the vicinity,
• wherein in a time period between receipt of a data frame sent from the contactless readable card unit (PICC) to the card reading device (PCD), which at least due to the received modulation, which is caused by the contactless readable card unit (PICC) via the variation of the high-frequency magnetic field, a has the correct structure, and a subsequent transmission of information from the card reader (PCD) to the contactless readable card unit (PICC), the magnetic field strength is reduced compared to an initial magnetic field strength to a predetermined lowered magnetic field strength, the initial magnetic field strength being that magnetic field strength with which that which the card reader did not modulate High-frequency magnetic field during a Information transfer between the card reader and the contactless readable card unit is generated. For example, after information has been transmitted to the contactless card unit (PICC) and a subsequent correct receipt of a response from the contactless card, which is caused by the contactless card unit (PICC) varying the high-frequency magnetic field, the magnetic field strength is compared to an initial magnetic field strength lowered, with which the part of the card reader is not modulated high-frequency magnetic field is generated during information transmission between the card reader and the contactless readable card unit.

A card reading device can thus save energy in those times when the contactless readable card unit is in a waiting state anyway and does not fully require the energy provided by the high-frequency magnetic field at the initial magnetic field strength. The card unit that can be read contactlessly is thus put into a low-energy mode in a targeted manner, in which only the data required for the communication connection has to remain stored. In the low-power mode, the card unit that can be read contactlessly does not have to be able to receive information from or send information to the card reader. Only minimal activity is required, e.g. B. resetting the contactless readable card unit, z. B. when the magnetic field strength is increased again, but triggers a reinitialization.

• eine Steuereinheit,
• eine mit der Steuereinheit verknüpfte steuerbare Sendeeinheit zum Erzeugen eines Hochfrequenzmagnetfelds in einem Nachbereich des Kartenlesegeräts, wobei eine Magnetfeldstärke des Hochfrequenzmagnetfelds steuerbar ist, um eine Kommunikation mit der kontaktlos auslesbaren Karteneinheit (PICC) auszuführen, und wobei die Sendeeinheit ausgebildet ist, für eine Informationsübermittlung von dem Kartenlesegerät zu einer in dem Nahbereich befindlichen kontaktlos auslesbaren Karteneinheit (PICC) eine Modulation des Hochfrequenzmagnetfelds auszuführen;
• und eine mit der Steuereinheit verknüpfte Auswerteeinheit zum Auswerten des Hochfrequenzmagnetfelds, um eine Variation des Magnetfelds durch die in dem Nahbereich befindliche kontaktlos auslesbare Karteneinheit (PICC) zu erfassen und zu analysieren,
• wobei die Steuereinheit (2) ausgebildet ist, in einem Zeitbereich zwischen einem Empfang eines von der kontaktlos auslesbaren Karteneinheit (PICC) zu dem Kartenlesegerät (PCD) gesendeten Datenrahmens, der zumindest aufgrund der empfangenen Modulation, welche über die Variation des Hochfrequenzmagnetfelds durch die kontaktlos auslesbare Karteneinheit (PICC) bewirkt ist, eine korrekte Struktur aufweist, und einer nachfolgenden Informationsübermittlung von dem Kartenlesegerät (PCD) an die kontaktlos auslesbare Karteneinheit (PICC) die Magnetfeldstärke gegenüber einer Ausgangsmagnetfeldstärke auf eine vorfestgelegt abgesenkte Magnetfeldstärke abzusenken, wobei die Ausgangsmagnetfeldstärke jene Magnetfeldstärke ist, mit welcher das seitens des Kartenlesegeräts nicht modulierte Hochfrequenzmagnetfeld während einer Informationsübermittlung zwischen dem Kartenlesegerät und der kontaktlos auslesbaren Karteneinheit erzeugt wird.

Furthermore, a card reading device is created for communicating with a contactless readable card unit in the vicinity of the card reading device, which comprises:

• a control unit,
• a controllable transmission unit linked to the control unit for generating a high-frequency magnetic field in a region close to the card reading device, wherein a magnetic field strength of the high-frequency magnetic field can be controlled in order to carry out communication with the contactless readable card unit (PICC), and wherein the transmission unit is designed for information transmission from the Card reader to perform a modulation of the high-frequency magnetic field to a contactless readable card unit (PICC) located in the vicinity;
• and an evaluation unit linked to the control unit for evaluating the high-frequency magnetic field in order to detect and to detect a variation of the magnetic field by the contactless readable card unit (PICC) located in the vicinity analyze,
• wherein the control unit (2) is designed, in a time period between receipt of a data frame sent from the contactless readable card unit (PICC) to the card reading device (PCD), the Card unit (PICC) is effected, has a correct structure, and a subsequent transmission of information from the card reader (PCD) to the contactless readable card unit (PICC) to lower the magnetic field strength compared to an initial magnetic field strength to a predetermined lowered magnetic field strength, the initial magnetic field strength being that magnetic field strength with which the high-frequency magnetic field that is not modulated by the card reader is generated during information transmission between the card reader and the card unit that can be read contactlessly.

• einen Empfangsschaltkreis um Empfangen eines von einem Kartenlesegerät erzeugten Hochfrequenzmagnetfelds,
• eine Gleichrichtereinheit zum Bereitstellen von elektrischer Energie, welche aus dem Hochfrequenzmagnetfeld extrahiert wird,
• eine Demodulationseinrichtung, um auf das Hochfrequenzmagnetfeld seitens des Kartenlesegeräts modulierte Informationen demodulieren zu können,
• einen steuerbaren Schalter zum Verändern einer Last oder einer Kapazität des Empfangsschaltkreises und gezielten Variieren des Hochfrequenzmagnetfelds auf diese Weise zum Übermitteln von Antwortinformationen an das Kartenlesegerät, sowie
• eine Logikeinheit zum Auswerten der empfangenen demodulierten Informationen und Erzeugen von Antwortinformationen
• wobei die kontaktlos auslesbare Karteneinheit in zwei sich hinsichtlich des Energiebedarfs unterscheidenden Betriebsmodi, einem Normalmodus und einem Niedrigenergiemodus, betreibbar ist.

A contactless readable card unit is also created, which includes:

• a receiving circuit for receiving a high-frequency magnetic field generated by a card reader,
• a rectifier unit for providing electrical energy which is extracted from the high-frequency magnetic field,
• a demodulation device in order to be able to demodulate information modulated on the high-frequency magnetic field by the card reader,
• a controllable switch for changing a load or a capacitance of the receiving circuit and selectively varying the high-frequency magnetic field in this way for transmitting response information to the card reader, and
• a logic unit for evaluating the received demodulated information and generating response information
• wherein the card unit that can be read contactlessly can be operated in two operating modes that differ in terms of the energy requirement, a normal mode and a low-energy mode.

Preferably, the card unit that can be read contactlessly can be put into a low-energy mode in which at least negotiated communication parameters and/or information specific to a current communication connection remain stored and can be used in subsequent operation in normal mode.

In order to be able to resume the exchange of information and data with the card unit that can be read contactlessly after the magnetic field strength has been reduced, the magnetic field strength is increased again to the initial magnetic field strength. In order to ensure that the card unit is fully operational again, one embodiment provides that the magnetic field strength is increased again to the initial magnetic field strength prior to the subsequent transmission of information from the card reader to the card unit that can be read contactlessly, and a modulation of the magnetic field to effect the transmission of information only begins after at least a predetermined reinitialization time period (T_re) has elapsed since the magnetic field strength was increased to the initial magnetic field strength.

A card reading device thus includes a time module with which the elapse of the reinitialization period can be monitored. The time module can thus be designed as a delay module or include one. This time module can be embodied in a control unit of the card reading device in hardware or in a combination of hardware and software.

In one embodiment of a card unit that can be read contactlessly, the switching device is designed to reinitialize the card unit, in particular the logic unit, when it is detected that the magnetic field strength has increased again to the previously received initial magnetic field strength, the reinitialization bringing the card unit that can be read contactlessly into a functional state offset, which corresponds to that before the transition to the low-power state. The contactless readable card unit can thus be used again after reinitialization as if the interim transition to the low-energy mode had not taken place. In this sense, a functionality of the overall system of card reader and contactless readable card unit is not restricted by the operating method according to the invention when using a card reader according to the invention and a contactlessly readable card unit according to the invention compared to those embodiments that are known from the prior art.

A preferred embodiment thus provides that the card unit, in particular its logic unit, is not reinitialized after detecting the renewed increase in the magnetic field strength, but is reset to the functional state before the magnetic field strength decreased, which is called reinitialization.

As a rule, the contactless readable card units, which draw their required energy from the high-frequency magnetic field, are designed in such a way that the voltage generated in a rectifier circuit is stabilized. In this case, some electrical energy is stored in a capacitor, for example. The contactless readable card unit can thus maintain its functions for a period of time after the magnetic field strength has dropped. However, to ensure that the contactless readable card unit has sufficient time after sending a message to carry out all necessary post-processing steps, e.g. to inform circuits coupled to the logic unit about an upcoming operation in low-power mode and to switch to low-power mode put, it is provided in some embodiments that the lowering of the magnetic field strength is delayed by at least a predetermined post-processing period (Δt_na) compared to the correct receipt of the information transmission from the contactless readable card unit to the card reader. I.e. between the correct receipt of the information transmission from the contactless readable card unit to the card reader and the reduction in the magnetic field strength, at least a predetermined post-processing time period (Δt_na) is allowed to elapse. The lowering is delayed by the post-processing period from the correct receipt of a response message.

In the case of a card reader, the time module or another time module can monitor the elapse of the wrap-up period. The time module or the further time module can each have or implement a delay circuit, which delays the drop in the magnetic field strength compared to the end of the correct reception of information from the contactless readable card unit.

Due to the stabilization of the voltage supply in the card units that can be read contactlessly, post-processing for switching to the low-energy mode and switching to the low-energy mode does not have to be initiated in all embodiments after each response information is sent. Some embodiments of card units that can be read contactlessly provide that the switching device is designed to bring about the switching to the low-energy mode as soon as response information is transmitted to the card reading device. Other embodiments provide that the switching device is designed to cause switching to the low power mode after a decrease in the magnetic field strength is detected. Thus, it is provided that the switching device is designed to bring about the switching to the low-energy mode as soon as response information is transmitted to the card reading device or after a drop in the magnetic field strength is detected.

According to one embodiment, the predetermined lowered magnetic field strength is determined such that the contactlessly readable card unit can be operated correctly in a low-energy mode at the predetermined lowered magnetic field strength.

With different card units that can be read without contact, the energy required to maintain the low-energy mode can vary. Likewise, the received magnetic field strength, i.e. the voltage induced in a receiving induction, depends on a variety of environmental conditions, such as temperature, the relative orientation of the contactless readable card unit to the card reader, a distance between the contactless readable card unit and the card reader, etc. It is therefore advantageous to negotiate a value for the reduced magnetic field strength. This therefore represents a parameter for lowering the magnetic field strength. Such parameters are also referred to as lowering parameters.

One embodiment therefore provides that the card reading device transmits a request for determining acceptable lowering parameters for the contactlessly readable card unit to the contactlessly readable card unit, and the card reader extracts at least one lowering parameter from the response information received, which contains information about the lowered magnetic field strength for the card unit that can be read contactlessly is acceptable, so that it can be operated correctly in the low-power mode, and the predetermined reduced magnetic field strength is set accordingly.

The negotiation can also take place, for example, similar to the negotiation of the so-called S(PARAMETER) according to the ISO/IEC14443-4/Amd 1 standard.

A negotiation can take place, for example, when a card unit signals that it is being supplied with too much energy. Some protocols provide status parameters to signal this from the card entity. Also a lack of an answer to a request can meaningfully trigger a negotiation of the lowering parameters. However, negotiation can also be carried out routinely, especially with mobile card readers.

One embodiment provides that one or more offered reduction parameters for the magnetic field strength of the card reader are specified to the contactless card unit with the request to determine the acceptable reduction parameters for the contactless card unit. The card unit, which can be read contactlessly, can thus recognize the possibilities offered by the card reader.

In one embodiment, one or more acceptable reduction parameters are determined from the response information from the contactless card unit (PICC), when used, the contactless card unit (PICC) can be operated correctly in low-power mode, and the magnetic field reduction is carried out by the card reader in such a way that the pre-determined reduced magnetic field strength is greater than or equal to the lowest acceptable magnetic field strength according to the acceptable reduction parameters of the contactless card reader (PICC). This ensures that a reduction takes place in such a way that the card unit that can be read contactlessly can be reliably and correctly operated in the low-energy mode.

It can be provided that in the course of the negotiation the reduction parameters selected by the card reading device are again transmitted to the contactless readable card unit and, if necessary, also confirmed by it again. This can be done in each case by transmitting the respective lowering parameters.

In addition to the magnetic field strength, the droop parameters may also include the post-processing period and/or the reinitialization period, and optionally additional parameters related to the droop and operation in the low-power mode. In one embodiment, it can also be negotiated that some control commands and responses, which are used in particular to monitor the ongoing presence of the contactlessly readable card unit, are still exchanged or can be exchanged. Such control commands can include data frames that contain a "not understood" (NAK-not acknowledged) and an "understood" (ACK-acknowledge) include. According to ISO/IEC 14443, such messages or data frames are referred to as R blocks (R blocks). If the post-processing period and/or the reinitialization period are also negotiated, a delay time can be optimized, in particular reduced, both when lowering and, in particular, when communication is resumed.

The card unit that can be read contactlessly cannot generally determine the magnetic field strength in absolute terms, so in one embodiment at least the reduction parameters that relate to the magnetic field strength are specified relatively, for example in the form of dividing factors, percentages or the like. The card reader can specify which reduction levels it can handle, for example 80%, 60%, 50%, 40%, 25% of the initial magnetic field strength. The contactless readable card unit can then use the high-frequency magnetic field currently received at the time of the negotiation, which has the output magnetic field strength, for example using the voltage induced in the receiving induction or in the receiving resonant circuit, to determine how much the magnetic field strength can be reduced relative to the received magnetic field strength in order to To be able to operate in low-power mode. Since the induced voltage scales linearly with the magnetic field strength, a reduction in the magnetic field strength by a reduction factor in the card reader leads to a reduction in the induced, i.e. received voltage, by the same reduction factor. Halving (reduction factor equal to ½) the magnetic field strength leads to a halving of the induced voltage in the receiving induction, i.e. the receiving antenna, the contactless readable card unit. One embodiment therefore provides that the reduction parameters for the magnetic field strength are specified in each case relative to the currently generated or received output magnetic field strength.

The card unit that can be read contactlessly can determine in different ways how much energy it is currently being supplied with and how much it needs relative to the current state for the low-energy mode.

Another embodiment can provide that the field strengths with which the contactlessly readable card unit can be operated are specified in absolute terms.

One embodiment of a card unit that can be read contactlessly provides that one or more reduction parameters are determined based on the received magnetic field strength, which are acceptable, so that when they are used by the card reader, correct operation of the card unit that can be read contactlessly in low-energy mode is possible, and this one or these multiple acceptable reduction parameters are transmitted to the card reader as part of a reduction parameter negotiation.

In order to prevent card units that do not master the low-energy mode from not being switched off, one embodiment provides that the card reading device checks whether several (in particular a second contactless readable card unit that does not master the low-energy mode) are present in the high-frequency magnetic field, which, for example, is tested according to ISO/IEC 14443 when processing a so-called anti-collision algorithm at the beginning or before the beginning of a communication connection. If several card units are present in the high-frequency magnetic field, the magnetic field strength is not reduced so that all card units, including those that may not be able to be operated in the low-energy-saving mode or cannot be operated under the same conditions, remain functional.

The magnetic field strength is preferably only reduced if the response received is a formally correct message according to a protocol used for communication, for example according to the ISO/IEC 14443-4 standard as a formally correct data frame or data frame, for example a data frame with a correct Application Protocol Data Unit (APDU) is recognized as the content. This prevents a dropping from taking place when the communication conditions are not optimal.

In some embodiments, the communication between the card reading device and the card unit that can be read contactlessly is preferably carried out at least on a physical level in accordance with the ISO/IEC 14443 standard. Based on this, various communication standards can be implemented.

In a preferred embodiment, the control unit of the card reader is designed in such a way that it controls all communication between the card reader and the contactless readable card unit controls. This means that the control unit also runs programs at high communication levels based on an OSI model.

Fig. 1

eine schematische Darstellung einer Ausführungsform eines Kartenlesegeräts;

Fig. 2

schematische Darstellung der Übermittlungsaktivitäten eines Kartenlesegeräts und einer kontaktlos auslesbaren Karteneinheit sowie einer von dem des Kartenlesegerät erzeugten Magnetfeldstärke jeweils aufgetragen gegenüber der Zeit; und

Fig. 3

ein schematisches Ablaufdiagramm eines Kommunikationsprozesses.

The invention is explained in more detail below with reference to a drawing. Here show:

1

a schematic representation of an embodiment of a card reader;

2

Schematic representation of the transmission activities of a card reading device and a card unit that can be read contactlessly, as well as a magnetic field strength generated by the card reading device, each plotted against time; and

3

a schematic flow chart of a communication process.

In the following description of the figures, the invention is described using exemplary embodiments which are based on the ISO/IEC 14443 family of standards. Cards that can be read without contact are described as card units that can be read without contact. Analogously, what has been described also applies to differently designed contactless readable card units in which the functionality is implemented, for example, in a mobile device that behaves like a contactless readable card in accordance with the ISO/IEC 14443 standard. What has been described also applies analogously to other near-field communication standards or variants of the ISO/IEC 1443 standard that are not explicitly described here.

In 1 a card reader 1 is shown schematically. The card reading device 1 includes a control unit 2 which controls the functioning of the card reading device 1 . The control unit 2 generally includes a microprocessor, a memory and software stored therein, which can be executed on the microprocessor. These sub-components of the control unit 2 are not shown for reasons of simplification. The control unit is designed to control a transmission unit 3, which is represented by a chain line. The transmission unit 3 is designed in such a way that it can generate a high-frequency magnetic field in the vicinity of the card reading device 1 .

Furthermore, the transmission unit 3 is designed in such a way that, controlled by the control unit 2, it can carry out a modulation of the magnetic field strength in order to transmit information. For this purpose, the transmission unit 2 comprises a modulation unit 4. The modulation unit 4 is designed, for example, in such a way that it can modulate the signal in accordance with the ISO/IEC 14443 Type B standard. In some embodiments, the modulation unit 4 can also be designed in such a way that it can also execute other modulation methods under the control of the control unit 2 . In order to be able to influence an unmodulated magnetic field strength a and a modulated magnetic field strength b, an impedance register for the modulated magnetic field strength 5 and an impedance register for the unmodulated magnetic field strength 6 are provided in the illustrated embodiment. A value of an output impedance register 7 is set by the modulation unit 4 according to the modulation via the values that can be defined via the control unit 2 . The value of the output impedance register 7 determines the output impedance of a driver unit 8. This amplifies a signal from an oscillator 9, which oscillates at a carrier frequency, for example 13.56 MHz, and generates a transmission signal. The transmission signal is sent to a transmission antenna 11 via a filter unit 10 . The transmission antenna 11 is preferably designed as a transmission induction with a conductor loop.

Other embodiments are also known in the prior art as to how the modulation strength or, more precisely, the magnetic field strength, can be regulated. Modern microchips use current control so that an output impedance of the driver unit remains constant, so that optimal impedance matching to the transmitting antenna or transmitting induction is always guaranteed.

In the prior art, a reception signal is additionally tapped at the transmitting antenna via a processing circuit, via which a variation of the generated high-frequency magnetic field can be detected by a contactless readable card (not shown) in the vicinity of the card reader during information transmission from the contactless readable card to the card reader .

In the embodiment shown here, an evaluation unit 12 is provided, which is shown by a dash-double-dot line. The evaluation unit 12 is coupled to a reception antenna 13, which is preferably designed as a reception induction with a conductor loop. The by the high-frequency magnetic field, which of of the transmitting unit 3 is generated, the induced high-frequency magnetic field signal is processed via a processing circuit 14 of the evaluation unit 12 for further processing. On the one hand, this received high-frequency magnetic field signal can be supplied to a demodulation unit 15, which performs demodulation in accordance with the ISO/IEC 14443 type B standard and outputs the information obtained to the control unit in the form of data. In addition, in the embodiment described here, the processed, received high-frequency magnetic field signal is supplied to a measuring circuit 16, which determines values representing a current magnetic field strength of the high-frequency magnetic field. It is understood by those skilled in the art that the magnetic field strength of the radio frequency magnetic field oscillates with the carrier frequency of the radio frequency magnetic field. The radio frequency magnetic field signal, in turn, oscillates at the frequency of the radio frequency magnetic field. A peak-to-peak value of the amplitude of the radio frequency magnetic field signal is a measure of the magnetic field strength of the radio frequency magnetic field. Thus, the high-frequency magnetic field signal can be viewed as a measure of the magnetic field strength of the high-frequency magnetic field. The measuring circuit 16 determines in each case a current value representing the magnetic field strength using the high-frequency magnetic field signal.

For this purpose, the voltage of the received high-frequency magnetic field signal is analyzed. Since amplitude shift keying is performed at a frequency that is lower than the carrier frequency of the high-frequency magnetic field, the magnetic field strength can be determined by averaging one or more half-waves of the induced magnetic field signal over time. It is advantageous to evaluate both the positive and negative half-waves separately. Such an averaging is carried out in a detection circuit 17, for example. This circuit is preferably designed as an analog circuit. A downstream analog/digital converter 18 provides the values for the determined magnetic field strengths for the control unit 2 . This is designed to use the received values to determine a measured modulation index from the values that were measured while the high-frequency magnetic field is unmodulated and modulated. The determined modulation index m _measured is compared with a specified modulation index m _{specification} . The comparison is advantageously carried out in such a way that it is checked whether the _measured modulation index m lies within a tolerance range around the specified modulation index. This tolerance range can, but does not have to, be symmetrical. If the determined modulation index is not within the tolerance range of the specified one modulation index, the control unit 2 changes the default values in the impedance register for the modulated magnetic field strength 5 and/or in the impedance register for the unmodulated magnetic field strength 6.

The default values for the modulation index and for the limiting values of the tolerance range Δt ₁ , Δt ₂ can be permanently specified or, in some embodiments, can be recorded or entered via an interface 19 . The interface 19 can also be used to exchange data that is to be transmitted to the card that can be read contactlessly or has been received by it. The interface 19 can be a communication interface or a user interface, for example in the form of a terminal, a touch screen with a graphical user interface, and so on.

By suitably selecting the measuring circuit 16 or detection circuit 17, the modulation index can be readjusted or controlled during a communication process in which data is transmitted to the contactlessly readable card. In other embodiments, however, provision is made for an adaptation sequence to be carried out as an alternative or in addition.

In preferred embodiments, the modulation and demodulation as well as the signal generation for a transmission signal is carried out in an integrated chip 20. This is indicated by a dotted line. In other embodiments, the measurement circuit 17 can also be integrated into the chip 20 . In such a case, the functionality of readjusting the modulation index can also be transmitted from the control unit to the chip. In other embodiments, the signal for the demodulation is not picked up at the reception antenna but at the transmission antenna. A separate processing circuit can be provided for this purpose. In still other embodiments, the processing circuit 14 between the receiving antenna 13 and the measuring circuit 16 and/or the demodulation unit 15 can be omitted.

The interface 19 is also used to couple the card reader to other devices, such as a computer or a computer network (both not shown) and the functionality of a system consisting of a card reader and a contactless readable card for applications such as Bank transactions, authentication of people for other processes, use of cryptographic keys that are stored on the contactless readable card, etc.

The control unit 2 is also designed to negotiate possible reduction parameters with the card that can be read contactlessly, and then to bring about a reduction in the magnetic field strength of the high-frequency magnetic field after a correctly recorded, transmitted response. In a preferred embodiment, the control unit 2 checks whether a transmitted data frame, which is also referred to as a frame, which contains a data unit of an application protocol, for example a so-called Application Protocol Data Unit (APDU) according to the protocol standard ISO/IEC 14443-4, has a corresponding answer is received. APDU are used as standardized content of a data frame. Data frames are often labeled with the content. A data frame that contains an APDU is therefore often simply referred to as an APDU. Data frames also contain I blocks and are therefore also referred to as I blocks or frames. In the following, an APDU, command APDU or response APDU is used to designate a data frame that contains an APDU. The terms "command" and "response" are used to distinguish requests from the card reader from responses from the contactless card. The control unit thus checks whether, after sending a command APDU to the card that can be read contactlessly, a correct response APDU, i.e. a correct response frame according to the protocol, was subsequently received. If this is the case and the data has been passed on to another application via the interface 19, the control unit 2 initiates a reduction in the magnetic field strength, which is suitable according to the previously negotiated reduction parameters in order to operate the contactless readable card correctly in a low-energy mode . The lowering can be achieved, for example, by setting the impedance register for the unmodulated magnetic field strength 6 accordingly.

An application data unit is a possible content of a data frame. If it is checked whether a correct data frame containing an APDU is present, a check according to the OSI model already takes place at layer level 4 or higher. In a simple embodiment, in which the content of the data frame that is received is not analyzed, it is only checked whether the structure of the received data frame, ie the number of bits, represents a correct data frame according to the protocol used for the information exchange. This also includes, for example, Verification that start and/or stop bits are correct, error correction at data link level, etc.

In 2 a time scale 101 is shown schematically at the top. Shown in the center of the figure is a plot of magnetic field strength 110 versus time. In the lower area, the transmission activities 120 of a card reading device (PCD) and below that a transmission activity 130 of a contactless readable card (PICC) are shown as binary representations, also in relation to the same time scale 101 . Various points in time required for an explanation of a sequence of a communication method or a method for operating a card reading device are indicated by vertical dashed lines. The corresponding points in time are identified by t1 to t7.

It is assumed that the section of the time scale shown represents an area for which a communication connection has already been established between the card reader and the contactless readable card and both the necessary parameters, which relate to a transmission rate and the like, and the reduction parameters used, which affect the reduction of the magnetic field strength, have already been negotiated between the card reader and the contactless readable card.

The high-frequency magnetic field generated by the card reader, which has a frequency of 13.56 MHz, for example, has an initial magnetic field strength 111, which is also referred to as H_int, ie as the initial field strength. A percentage value of 100% is assigned to this initial magnetic field strength. This output magnetic field strength 111 is the magnetic field strength that is generated by the card reading device during communication without a modulation for information transmission being undertaken by the card reading device or by the contactless readable card. In the schematic representation of 2 Influences of the modulation for information transmission on the magnetic field strength 110 are not shown.

It is further understood by those skilled in the art that the magnetic field strength of the radio frequency magnetic field oscillates with the carrier frequency of the radio frequency magnetic field. However, an envelope of the maximum values of this oscillation is shown. if when the magnetic field strength is mentioned, an effective value or a value that is described by such an envelope is always meant.

At time t1, the card reading device (PCD) sends a data frame containing an application protocol data unit (APDU) to the contactless readable card (PICC). The transmission ends at time t2. The contactless card (PICC) responds to this so-called command or command APDU with a response frame that contains a response APDU. This transmission starts at time t3 and ends at time t4. After the card reading device (PCD) has correctly received the response from the contactless readable card (PICC), it waits for a predetermined, in particular negotiated, post-processing period Δt_na until it reduces the magnetic field strength at time t5 to a lowered magnetic field strength 112, which in the example shown is 50%. the output magnetic field strength is 111, lowers.

The contactless readable card (PICC) is switched to a low-energy mode either after the response APDU has been sent at time t4 or after the magnetic field strength 110 has been reduced at time t5. In this low-energy mode, however, the card that can be read without contact must no longer be able to demodulate or detect modulations in the frequency magnetic field and otherwise no longer provide any functionality that goes beyond storing and retaining those data that are required for the smooth continuation of the existing communication connection are necessary at a later date. One functionality is therefore that the contactless readable card (PICC) must be able to recognize that the communication connection is not interrupted, but is only continued in low-power mode by the contactless readable card. In addition, the contactless readable card has the ability to detect a renewed increase in the magnetic field strength to the initial magnetic field strength 111 and to restore the contactless readable card (PICC) to a functional state as it was before the magnetic field strength dropped at time t4 or t5 prevailed.

If the card reader (PCD) receives a request to continue communicating with the contactless readable card (PICC) or if it wants to do this of its own accord, it increases the magnetic field strength 110 of the high-frequency magnetic field again responds to the output magnetic field strength 111 as shown at time t6. In order to give the contactless readable card the opportunity to reset itself to the fully functional state, i.e. to be reinitialized, the card reader (PCD) waits for a reinitialization period Δt_re until there is a renewed transmission of data in a data frame, for example with another application protocol data unit , ie, for example, a command APDU as content, as indicated at time t7.

The method described here for operating a card reader or a card that can be read contactlessly can be inserted into any higher-level transmission method and be a component of such a method and has the effect that the transmission energy required can be significantly reduced. Particularly in applications in which higher-level processes and devices coupled to the card reader process data exchanged between the card reader (PCD) and the contactless readable card (PICC), long times or time ranges can occur in which the Communication connection is virtually only maintained and in which no content-related data exchange takes place between the card reader (PCD) and the contactless readable card (PICC). In the exemplary embodiment shown, this is a time range 140 between times t4 and t7 or t5 and t6, taking into account that there is already a request for renewed communication with the contactless readable card at time t6. As a rule, however, unlike the representation of the figure 2 suggests that the time span between the times t5 and t6 be considerably greater than the post-processing time span Δt_na and the reinitialization time span Δt_re.

In some embodiments, the post-processing period Δt_na can be greatly shortened or omitted since only a short time is required to post-process a transmission and/or the energy supply of the card unit that can be read without contact has a high buffer capacity for storing electrical energy. Post-transmission processing and low-power mode switching can be performed with the stored energy.

In the following, the negotiation of lowering parameters, in particular the lowering parameters for the lowered magnetic field strength, will be discussed. On the ordinate, ie the magnetic field strength scale 150 of the middle graph in 2 are up the left-hand side as offered reduction parameters 160 offered reduction levels 161 to 165 drawn by means of empty circles, which correspond to those reduction levels which the card reader can generate in relation to the output magnetic field strength. These are 80% for the offered reduction level 161, 60% for the offered reduction level 162, 50% for the offered reduction level 163, 30% for the offered reduction level 164 and 20% for the offered reduction level 165. The percentages are based on the output magnetic field strength 111 based. These possible reduction levels 161 to 165 or reduction factors are transmitted from the card reader to the contactless readable card (PICC) during negotiation.

The contactless readable card is able to operate correctly in the low-power mode when the output magnetic field strength is reduced to 75% for the acceptable reduction level 171, 60% for the acceptable reduction level 172, or 50% for the acceptable reduction level 173. The acceptable lowering parameters 170 of the contactlessly readable card are thus indicated by empty diamonds to the right of the magnetic field strength scale 150 . These lowering levels are transmitted back to the card reader as acceptable lowering parameters from the contactless readable card. In some embodiments, only the offered reduction parameters are transmitted back to the card reader as acceptable reduction parameters, in which there is a match between the offered reduction parameters transmitted by the card reader to the contactless card, here the offered reduction parameters or offered reduction levels 162, 163, for which matching acceptable Reduction parameters of the card that can be read contactlessly, here the acceptable reduction stages 172, 173, exist. The card reader selects the lowest possible dip, i.e. those dip parameters that result in the lowest magnetic field strength of the radio frequency magnetic field consistent with the contactless readable card (PICC) acceptable dip parameters 170, so that the contactless readable card (PICC) operates correctly in low power mode can be. The circles thus indicate the reduction levels or reduction parameters 160 offered with regard to the magnetic field strength, ie the relative magnetic field strengths offered, and the diamonds displayed to the right of the magnetic field strength scale 150 indicate the acceptable reduction parameters 170 or acceptable relative magnetic field strengths.

In 3 a flow chart of a method for operating a card reading device is described as an example. Again, it is assumed that a communication link has already been established between the card reader and the card that can be read contactlessly, in order to make data from the card that can be read contactlessly readable for an external application usable. In a method step 201, the card reading device sends reduction parameters to the contactless readable card. The reduction parameters may include offered reduction levels, offered wrap-up periods, and offered reinitialization periods. The contactless card responds by sending the acceptable reduction parameters. The acceptable droop parameters may include acceptable droop levels, required and/or acceptable post-processing time periods, or required and/or acceptable reinitialization time periods. The card reader selects the magnetic field strength according to the lowest offered reduction level that is compatible with the lowest acceptable reduction level as the predetermined reduced magnetic field strength 203. Furthermore, the card reader sets a predetermined reinitialization period according to the exchanged reduction parameters 204. Similarly, the card reader proceeds with the reinitialization period and sets it accordingly the exchanged lowering parameters fixed 205. Then there is a transition to an operating state in which the content of data is exchanged between the card reader and the contactless readable card. In method step 206 it is checked whether a so-called command APDU has been transmitted by an application. If this is not the case, a branch is made back to this query 207. A command APDU is viewed here as representative of any information to be transmitted in a data frame. If a command APDU was transmitted from an external application to the card reader, it is checked whether the magnetic field strength of the high-frequency magnetic field matches the output magnetic field strength 209. If this is not the case, the magnetic field strength is increased to the output magnetic field strength according to branch 210 211 and then a Waiting for a reinitialization period of time 212. Then, as with branch 213 of query 209, ie if the output magnetic field strength is already present, the high-frequency field is modulated in order to send the command APDU to the contactless readable card 214. Then the high-frequency magnetic field is checked for variations analyzes which are caused by the contactless readable card, and extracts a response APDU and sends it to the external application forwarded 215. In query 216, it is checked whether a correct response APDU was received after a command APDU. A response APDU is also only used here as an example of information that is or can be transmitted in a data frame. If a correct response APDU has been received, branch 217 waits for a post-processing period 218 and then the magnetic field strength is lowered to the predetermined lowered magnetic field strength 219. The method then continues with method step 206, in which it is checked whether a new command -APDU is transmitted by the external application. If a correct response APDU was not received and this was determined in method step 216, then, according to the no branch 220, the method also continues with method step 206.

It will be understood by those skilled in the art that only exemplary embodiments are described here. The various features described can be used in any combination to implement the invention. Deviations are possible both with regard to the hardware and with regard to the order of the process steps.

Reference List

1

card reader

2

control unit

3

transmitter unit

4

modulation unit

5

Impedance register for the modulated magnetic field strength

6

Impedance register for the unmodulated magnetic field strength

7

Output Impedance Register

8th

driver unit

9

oscillator

10

filter unit

11

transmitting antenna

12

evaluation unit

13

receiving antenna

14

conditioning circuit

15

demodulation unit

16

measuring circuit

17

detection circuit

18

A/D converter

19

interface

20

integrated chip

101

timescale

110

magnetic field strength

111

output magnetic field strength

112

reduced magnetic field strength

120

Card reader sending activity

130

Card reader sending activity

140

time range

150

magnetic field strength scale

160

offered reduction parameters

161-165

offered reduction levels

170

acceptable drawdown parameters

171-173

acceptable reduction levels

200

Method of operating a card reader

201 - 220

Process steps/branches

Claims (14)

1. Method (200) for operating a card reader (1), comprising the steps of:

   - generating a high-frequency magnetic field in a close range of the card reader (1),

   wherein the high-frequency magnetic field is controlled in order to perform a communication with a contactlessly readable card unit, and

   wherein a transmission of information from the card reader (1) to the contactlessly readable card unit situated in the close range prompts performance of a modulation of the high-frequency magnetic field;

   - evaluating the high-frequency magnetic field in order to capture and analyse a variation in the high-frequency magnetic field as a result of the contactlessly readable card unit situated in the close range,

   characterized in that

   in a time range between a reception of a data frame transmitted from the contactlessly readable card unit to the card reader, the data frame having a correct structure at least due to the received modulation that is caused by the variation in the high-frequency magnetic field by the contactlessly readable card unit, and a subsequent transmission of information from the card reader to the contactlessly readable card unit, the magnetic field strength (110) of the non-modulated high-frequency magnetic field generated in the close range of the card reader (1) is lowered in comparison with an initial magnetic field strength (111) to a magnetic field strength of the non-modulated high-frequency magnetic field that is lowered in a predefined manner, such that the high-frequency magnetic field is generated at the lowered magnetic field strength (112) in the close range of the card reader (1),

   wherein the initial magnetic field strength (111) is that magnetic field strength at which the high-frequency magnetic field that is not lowered and not modulated by the card reader is generated during the information transmission between the card reader and the contactlessly readable card unit,

   wherein the predefined lowered magnetic field strength (112) of the non-modulated high-frequency magnetic filed is determined so that the contactlessly readable card unit is able to be operated in a low-energy mode at the predefined lowered magnetic field strength (112),

   wherein in the low-energy mode the communication connection is not interrupted but continued only in the low-energy mode by the contactlessly readable card unit, wherein in the low-energy mode the contactlessly readable card unit, however, does not have to be able to receive information from the card reader or transmit information thereto,

   wherein, however, the contactlessly readable card unit stores at least transmission parameters that are negotiated for a current communication connection and/or other data specific to the communication connection in the low-energy mode in such a way that these are subsequently available again in a normal mode without the need for renegotiation of the parameters or data.
2. Method (200) according to Claim 1, characterized in that the magnetic field strength (110) of the non-modulated high-frequency magnetic field is increased to the initial magnetic field strength (111) again before the subsequent information transmission from the card reader to the contactlessly readable card unit and the modulation of the high-frequency magnetic field for effecting the information transmission begins only after at least one predetermined reinitialization period has passed since the increase in the magnetic field strength (110) to the initial magnetic field strength (111).
3. Method (200) according to either one of the preceding claims, characterized in that the lowering of the magnetic field strength (110) is delayed by at least one predetermined post-processing period with respect to the correct reception of the data frame transmitted from the contactlessly readable card unit to the card reader.
4. Method (200) according to any one of the preceding claims, characterized in that the card reader transmits to the contactlessly readable card unit a request for determining lowering parameters (170) that are acceptable for the contactlessly readable card unit and the card reader extracts from an obtained piece of response information at least one lowering parameter containing an indication of which lowered magnetic field strength (112) of the non-modulated high-frequency magnetic field is acceptable for the contactlessly readable card unit so that it is able to be operated in the low-energy mode, and the card reader accordingly determines the predetermined lowered magnetic field strength (112) of the non-modulated high-frequency magnetic field.
5. Method (200) according to Claim 4, characterized in that, together with the request for determining the lowering parameters (170) that are acceptable for the contactlessly readable card unit to the contactlessly readable card unit, one or more provided lowering parameters (161-165) for lowering the magnetic field strength of the non-modulated high-frequency magnetic field of the card reader are provided.
6. Method (200) according to Claim 5, characterized in that one or more acceptable lowering parameters are determined from the obtained response information of the contactlessly readable card unit in response to the request to determine the acceptable lowering parameters (170), the use of said acceptable lowering parameters enabling operation of the contactlessly readable card unit in the low-energy mode, and the card reader lowers the magnetic field so that the predetermined lowered magnetic field strength (112) of the non-modulated high-frequency magnetic field is greater than or equal to the lowest acceptable magnetic field strength of the non-modulated high-frequency magnetic field according to the acceptable lowering parameters of the contactlessly readable card unit.
7. Method (200) according to any one of Claims 4 to 6, characterized in that the lowering parameters indicate the magnetic field strength of the non-modulated high-frequency magnetic field in each case relative to the generated or received initial magnetic field strength.
8. Method (200) according to any one of the preceding claims, characterized in that the card reader checks whether a second contactlessly readable card unit is also present in the high-frequency magnetic field and, if this is the case, desists from the lowering of the magnetic field strength (110) of the non-modulated high-frequency magnetic field.
9. Card reader (1) for communication with a contactlessly readable card unit in a close range of the card reader (1), comprising:

   - a control unit (2),

   - a controllable transmission unit (3) linked to the control unit (2) for generating a high-frequency magnetic field in a close range of the card reader (1), wherein a magnetic field strength of the high-frequency magnetic field may be controlled in order to communicate with the contactlessly readable card unit, and wherein the transmission unit (3) is designed to modulate the high-frequency magnetic field for information transmission from the card reader (1) to a contactlessly readable card unit in the close range;

   and an evaluation unit (12) linked to the control unit (2) for evaluating the high-frequency magnetic field in order to detect and analyse a variation in the magnetic field by way of the contactlessly readable card unit in the close range,

   characterized in that

   the control unit (2) is designed to carry out the method according to any one of Claims 1 to 8.
10. Contactlessly readable card unit, comprising:

    - a reception circuit for receiving a high-frequency magnetic field generated by a card reader,

    - a rectifier unit for providing electrical energy that is extracted from the high-frequency magnetic field,

    - a demodulation device in order to be able to demodulate modulated information onto the high-frequency magnetic field on the part of the card reader,

    - a controllable switch for changing a load or a capacitance of the reception circuit and varying the high-frequency magnetic field in a targeted manner in this way to transmit response information to the card reader,
    and

    a logic unit for evaluating the received demodulated information and generating response information, characterized in that

    the contactlessly readable card unit is able to be operated in two operating modes that differ in terms of the energy requirement, a normal mode and a low-energy mode,

    wherein a switchover device is designed to set the contactlessly readable card unit to the low-energy mode, wherein at least negotiated communication parameters and/or information specific to a current communication connection remain stored in the low-energy mode and are able to be used in a subsequent operation in the normal mode and the communication connection is not interrupted but continued only in the low-energy mode by the contactlessly readable card unit,

    wherein in the low-energy mode the contactlessly readable card unit, however, does not have to be able to receive information from the card reader or transmit information thereto.
11. Contactlessly readable card unit according to Claim 10, characterized in that the switchover device is designed to reinitialize the card unit when it is determined that the magnetic field strength increases again to the received initial magnetic field strength, the reinitialization setting the contactlessly readable card unit to a functional state that corresponds to that before the transition to the low-energy state, wherein the received initial magnetic field strength is the field strength that has been received by the contactlessly readable card unit as magnetic field strength of the non-modulated high-frequency magnetic field on the part of the card reader during the information transmission in the normal mode.
12. Contactlessly readable card unit according to either one of Claims 10 or 11, characterized in that the switchover device is designed to set the low-energy mode as soon as a piece of response information is transmitted to the card read or after a lowering of the magnetic field strength is detected, provided the lowering is greater than a change in the magnetic field strength caused by modulation for the information transmission on the part of the card reader.
13. Contactlessly readable card unit according to any one of Claims 10 to 12, characterized in that the switchover device is designed to determine, based on the received magnetic field strength of the high-frequency magnetic field, one or more lowering parameters that are acceptable, such that it is possible to operate the contactlessly readable card unit in the low-energy mode when said lowering parameters are used by the card reader and it is possible to transmit said one or more acceptable lowering parameters to the card reader within a lowering parameter negotiation.
14. System made up of a card reader and a contactlessly readable card unit, wherein the card reader and the contactlessly readable card unit are designed to carry out the method according to at least one of Claims 1 to 8 .

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